thanks for all the replies. Its probably not strictly a MIDI or signal path problem because i have the same issue with wave forms that are definitely in stereo. Im thinking of downloading ASIO drivers to see if that helps. I attached a screenshot of my master I/O in case theres anything there that might be causing the mono signal. Notice i panned both the master and the individual track pan to the right and took the screenshot at the same time the note was playing so you can see only the right channel is showing a signal as it should when the note is panned to right.
It may be your internal soundcard, but could also be related to your headphones. A bad headphone cable can cause similar issues, or if you are using an adapter to reduce from a quarter inch headphone plug, to an 1/8th inch(computer size) plug,make sure your adapter is a stereo adapter. I know this is trivial stuff, but even the experienced engineers overlook the small stuff from time to time.
I was just about to suggest lighting candles and chanting to clear away the angry spirits. Congratulations romanled. I know that feeling when you get it running after something inexplicable happens.
When I was using Adobe Audition the setup I'm using works (stereo) either thru Audiobox USB or computer speakers (i.e. panning left/right work). Using Reaper the output is mono only. I'm using the sample project "All Through The Night". Pan on track or master has no L/R effect other than a very slight volume dip on both channels. tried: When I assign options/preferences/Audio device Output range "first" to 2:OUT 2 the sound is all in right ear. Same for 1:OUT 1, all in left ear. Something in Reaper is MONO. What the heck is it? I have re-started Windows, uninstalled, re-installed Reaper. No change